Glicentin is a peptide comprising 69 amino acid residues, regardless of origin, as a major component of gut glucagon-like immunoreactants which are also called gut glucagon-like immunoreactivities (gut GLIs). The peptide contains the entire sequence of glucagon in positions 33-61 which is extended at the amino terminus via Lys-Arg with the 1-30 sequence, glucagon-related pancreatic peptide (or glicentin-related pancreatic peptide) and at the carboxy terminus via Lys-Arg with a hexapeptide (positions 64-69) (Volume 11 Gastrointestinal Hormones edited by V. Mutt, Noboru Yanaihara and Chizuko Yanaihara, pp. 141-162, Academic Press, Inc. 1988).
L. Thim & A. J. Moody have established the primary structure of porcine glicentin (Regulatory Peptides, 2 (1981), 139-150). S. Seino et al have suggested the amino acid sequences of human, bovine, hamster, rat and guinea pig glicentin from their preproglucagon sequences (FEBS, Vol. 203, No. 1, pp. 25-29, 1986). Glicentin and glucagon are produced by tissue specific processing from the same precursor, preproglucagon. Glucagon is formed in pancreas and glicentin in intestine. It is known that glucagon counter-acts the blood glucose-lowering action of insulin by stimulating glyconeogenesis and glycogenolysis (R. Ebert et al., Diabetes Metabolism Review, Vol. 3, No. 1, 1-26 (1987)). Glucagon-like peptide-1 (GLP-1) (1-37) produced by tissue specific processing from the same preproglucagon is a 37-amino acid polypeptide hormone and the peptide derived from it, GLP-1 (7-36 amide) is a 30 amino acid polypeptide hormone. Those hormones are known to have an insulin releasing function (T. Matsuyama et al., Diabetes Res. and Clinical Practice, Vol. 5, 281-284 (1988), D. A. D'Alessio et al., Diabetes, Vol. 38, 1534-1538 (1989)).
Some investigators reported that a fraction of Peak II (rich in glucagon of low molecular weight) prepared by fractionation of glucagon-like immunoreactivity (GLI) extracted from the mucosa of small intestine in dogs exhibited the stimulation of insulin secretion, whereas a fraction of Peak I (rich in glicentin) did not exhibit it (A. Ohneda et al., Horm. Metab. Res. Vol. 8, 170-174 (1976)).
On the other hand, the biological action of glicentin has not been confirmed. It is unknown on what tissue or cell glicentin acts directly. Glicentin so far isolated and purified is an origin of other mammalian animals than humans. Human glicentin has not been isolated as a purified product because of the difficulty in an availability of the materials for extraction, i.e., human gut. Thus, the physiological roles of human glicentin have not been elucidated.
The present inventors were successful in synthesizing DNA corresponding to the amino acid sequence of human glicentin which was deduced by Graeme I. Bell et al. (Nature, Vol. 304, 368-371 (1983)) from the sequence of human preproglucagon gene, preparing a recombinant DNA vector using the synthesized DNA and then producing human glicentin from a host cell transformed by the recombinant DNA (Japanese Patent Kokai Hei 4-364199). This success leaded to easy availability of human glicentin in a large amount and as a purified product.
With a rise in the standard of living, the number of diabetic patients is yearly increasing. The prevalence for the past 30 years shows a rapid increase tendency as much as 30 times or more. The morbid state of diabetes will be caused by an absolute or relative lack of an insulin function which plays a central part in the regulation of blood glucose. The main method for the treatment of diabetes includes an alimentary therapy and an administration of insulin.
Sulfonylurea is known as a drug for stimulating insulin secretion. However, this drug has the disadvantages in that over- or continuous-administration leads to a risk of causing hypoglycemia and enough attention is required to keep a normal blood glucose level. For the patients suffering from hepatopathy and nephropathy, a special care is further required in the administration, since the drug or its metabolite is accumulated. In addition, an administration of the drug to gravida is not recommended because of its placental passage and an administration to a nursing woman is impossible because of its easy migration to milk. Therefore, there is a desire to develop an insulin secretomotory agent having high safety and less side effects.